ppc40x: Remove unmaintained target

ppc40x: Remove unmaintained target

This target only supports kernel 3.18, which is not supported in OpenWrt
any more for multiple releases. It also looks like there is no active
maintainer for this target.
Remove the code and all the packages which are only used by this target.

To add this target to OpenWrt again port it to a recent and supported
kernel version.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>